Lifting devices have been used for many years to conveniently dump a load from a vehicle. The lifting devices are generally mounted between the truck bed and chassis to convert a pickup truck into a small dump truck. Activation of the lifting device tilts the truck bed about a pivot point thereby dumping the contents of the truck bed at a desired location. There are many instances where a small dump truck or dump trailer is desirable. For example, when performing routine maintenance and other projects, municipalities frequently require relatively small amounts of sand, gravel or asphalt to be delivered to a site. Also, landscapers, gardeners, and home owners have use for a dumping vehicle for lawn care and other home repair projects. A dual pickup/dump truck reduces costs and increases efficiency by allowing multiple uses of trucks already being used for other purposes.
All lifting assemblies require the application of force to lift a load. Such force may be applied by supplying an incompressible fluid (air, hydraulic fluid, etc.) under pressure to an expandable device positioned between the load an vehicle chassis. For an efficient system, it would be desirable to utilize all the pressure available to lift the load throughout the entire lift cycle.
To dump a load, the tilting mechanism must rotate and lift the bed about an axis approximately 45-50.degree. from horizontal to allow the material contained therein to flow by gravity from the bed. The weight of the load (W.sub.load), acting downwardly at its centroid, multiplied by the perpendicular distance between the force and the bed pivot (D.sub.load) gives the load moment (M.sub.load) necessary to raise the bed. Written as an equation: M.sub.load =W.sub.load.times.D.sub.load. The lifting moment (M.sub.lift) is the force of the lifting component attached to the bed (F.sub.lift) multiplied by the perpendicular distance from the pivot to the line of force of the lifting component (D.sub.lift). Written as an equation: M.sub.lift =F.sub.lift.times.D.sub.lift. It is desirable to have the maximum available lifting moment (M.sub.lift) equal the load moment (M.sub.load).
Generally, there are two types of lifting assembles: inflatable lifting bags and hydraulic operated dumping mechanisms. Hydraulically driven lifting assemblies provide a significant amount of force to dump pickup beds. In general, however, installation of this equipment is cumbersome, difficult, and expensive. Furthermore, installation of hydraulic cylinder dumping assemblies requires tools not generally available to the homeowner, resulting in decreased sales due to increased costs and shipping difficulties. These types of tilting mechanisms, generally, require raising the truck bed vertically several inches to provide room for mounting the tilting apparatus. Raising the truck bed, however, causes misalignment of the lines and pin-striping between the cab and bed thereby reducing the aesthetic qualities of the vehicle. As the overall appearance of the truck is very important to the consumer, many owners of pickup trucks are unlikely to buy and install hydraulic lift equipment.
There are several mechanical disadvantages associated with the aesthetic and cost concerns of hydraulically driven lift assemblies described above. Hydraulic cylinder actuated devices are inherently inefficient as it is difficult to match the load moment to the lifting moment because the hydraulic cylinder must maintain the same cross section. The load moment is set by the weight and geometry. The only variables in the lift moment are the lift force, which is constant for a specified cylinder pressure, and the distance (D.sub.lift) from the line of action of the force to the pivot. As disclosed in U.S. Pat. No. 5,513,901, some advancements have been made in the prior art with various types of cams and linkages to vary the distance such that the lifting moment will more closely match the load moment. However, this arrangement requires complex and expensive mechanisms that will not mount in the limited space available under the truck bed necessary to maintain the vehicle's aesthetic integrity. Furthermore, hydraulic lifting devices require the use of a hydraulic fluid, that can cause harm to the environment if leaked or spilled.
In addition to hydraulic lift assemblies, various inflatable bag assemblies have been employed to more effectively and efficiently lift and tilt truck beds. Some of these prior art vices are disclosed in U.S. Pat. Nos. 3,659,899 to Phillips et. al, 3,711,157 to Smock, 3,784,255 to Smock, 5,067,774 to Trowland, and 5,560,684 to Gilmore.
For example, U.S. Pat. No. 5,067,774 to Trowland discloses a truncated cone shaped bag that allows the bag to deflate in a more controlled manner. The Trowland bag, however, is not optimized to use constant pressure during the lifting cycle, or to fold along the path of travel of the upper thrust plate, thereby resulting in a less effective and less efficient lifting assembly. Another prior art bag lifting assembly is disclosed in U.S. Pat. No. 5,560,684 to Gilmore. The patent to Gilmore discloses a two-stage lifting assembly. The first stage includes walls that extend substantially at right angles to the chassis for tilt angles of 0.degree. to 20.degree., while the second stage bag is a truncated right cone, similar to the entire bag design in the patent to Trowland. As stated in Gilmore, the purpose of the two stage bag arrangement is to provide thrust at generally right angles to the tipping tray irrespective of the tipping angle. However, several disadvantages are apparent in the patent to Gilmore that limit its effectiveness. First, although it might be considered an advancement, in some ways, over the lifting assembly in Trowland, the lifting bag in Gilmore is not optimized for the available pressure. In addition, like the patent to Trowland, the bag disclosed in the patent to Gilmore is not optimized to fold along the path of travel of the upper thrust plate. Also, the bag disclosed in the Gilmore patent will tend to buckle from an eccentric loading on the knee portion between the two bag stages.
Furthermore, many of the prior art lifting bag assemblies utilize exhaust gases from the engine to activate the bag. One of the many disadvantages of this arrangement includes the voiding of most manufacturers'engine warranties due to back pressure from the exhaust. In addition, heat from the exhaust gas is detrimental to the elastomer in the bag, and thereby requires a complex and expensive cooling system to cool the gas before entering the bag. In addition, the use of exhaust powered bags requires the addition of a diverter valve to divert the exhaust into the bag along with a separate bypass valve to bypass the exhaust gas from the bag to avoid engine stalling. Generally, this arrangement results in an inefficient and leaky system that requires the engine not only to be running to initially raise the load, but to also maintain the load in its raised position. The exhaust powered system also makes it virtually impossible to stop the lift at a midcycle position. Furthermore, the usable pressure of the exhaust powered system is limited by the back pressure that the engine will allow without stalling.
In addition to that described above, the inflatable bag lifting assemblies of the prior art all have the disadvantage of providing an inflatable bag that allows for rotation with respect to both the top and bottom thrust plates, thereby increasing the tendency of the bag to buckle during inflation and deflation. Furthermore, because the prior art bag is allowed to rotate during inflation and deflation, the air inlet to the bag necessarily moves with the bag as the bag rotates on the thrust plate. Accordingly, it is difficult to firmly secure or tie down all of the inlet equipment and connecting attachments, including the hose, while also providing flexibility in the connections to accommodate the movement of the bag during activation and deactivation.
Although some advancement has been made in the prior art to more effectively lift a load, the prior art lifting assemblies still have a number of drawbacks, such as effectively collapsing or folding into an acceptable length, width, or height so that the aesthetic appearance of the truck is preserved. Accordingly, even small advancements made in the prior art to more efficiently lift a load or reduce the mounting height of the truck bed have been considered significant in the production of lifting assemblies. As a result, to manufacture a lifting assembly desirable to the consumer, it is essential to fully utilize lifting pressure to lift the greatest load, minimize the bulk of the bag, and stabilize the bag upon inflation and deflation.